1. Field of the Invention
In the area of photovoltaics there is a constant push for greater efficiencies. Typical commercially available solar cell efficiencies vary anywhere from 10% for polycrystalline cells to as high of 25% for single crystal. In order to increase the energy output of the cells, manufacturers use glass substrates with higher transmittance than normal float glass. This increases the transmittance by approximately 2%. If the transmittance could be increased further, it would be a boon to the solar cell industry, lowering the cost per watt and thereby making solar power more affordable.
2. Discussion of the Art
We have long known from the work of Fresnel that the reflection from the surface of glass is controlled by the difference between the refractive index of the glass and the refractive index of air according to the following formula: ((n-1)/(n+1)).sup.2, where n is the refractive index of the glass and the refractive index of air is one. For most commercial glasses n=1.52, which means that you have approximately 4% of light reflected from the air glass interface. See FIG. 1. If there was no absorption of the light energy by the body of the glass 96% of the incoming light energy would pass through the glass. However, when the light emerges from the glass and again passes into air (n=1) light is reflected such that only 92.16% of the total energy emerges in the light. (See FIG. 2). For most ordinary purposes this surface reflectance is of no significance. However, in the case of photovoltaics which have relatively low efficiencies in converting light to electricity, recovering any of this lost light due to reflection would prove to be a benefit.
One way to reduce the reflectance from a given surface is to somehow form a layer of material of a lower refractive index on it. Again from Fresnel's work we see that if a layer having a refractive index of 1.24 is placed on the surface of a material having a refractive index of 1.52 a reduction in reflection can be achieved such that as shown in FIG. 3. Such a layer system can and has been achieved in many ways over the past few decades. Examples include:
Fluoride Coatings Involving the Use of Alkali Fluorides
Some of the earliest low reflectance coatings involved placing a film of magnesium fluoride which has a refractive index of 1.38 on the surface of glass. The processing is complex and expensive and the films are not all that strong.
Etched Films
People have also used a controlled etching process involving the use of hazardous chemicals. Long processing times are required and the resultant films are easily damaged. This process works by removing constituents of the glass and leaving a skeletal film of silica where the refractive index of the total film is a volume average of the silica skeleton and the air in the interstices. These films are also very delicate.
Porous Films
These skeletal films are produced without etching by using a sol-gel to form a film of silica particles on the surface of glass such that the refractive index of the film is an average of the refractive index of the silica particles and the air inbetween them. These films by nature are also very delicate and the application process is very exacting.
Multilayer
Additional layers of glasses with various refractive indexes fused to the surface with a second layer of a material with a lower refractive index. This process involves very high heat.
Sputter Coating
A complex process which is expensive in terms of power required, materials and handling.
All of the above films, while very good low reflectance films, require that they be manufactured in separate operations prior to solar cell manufacture. These processes involve the use of equipment to generate high temperatures, high vacuum or use hazardous materials to create the films.
What is needed is a low cost process that creates a low reflectance coating on one surface of the glass that can be used in an existing production line for glass or solar cell manufacture with a minimum amount of trouble or expense. Through the innovative combination of commercially available chemicals and the process of chemical grafting such a coating has now been developed.